Implantable medical devices such as pacemakers, defibrillators, speech processors, left ventricular assist devices (LVAD), and neurostimulators are becoming more and more common and have many unique requirements. Small, round, lightweight devices having flat shapes are desirable for ease of implant and patient comfort. Additionally, these devices have strict safety and reliability requirements.
In many of these devices, the batteries make up the majority of the weight and volume. Therefore, batteries are needed of a specific shape to make the best use of the space available in each device. For many of these devices, the batteries themselves should be small, flat, and lightweight.
One method of producing flat cells is by using a stacked plate design. As shown in FIG. 1, a typical battery stack 10 is constructed such that the positive electrodes 11 have tabs 12 that extend past the edges of the negative electrodes 13 and serve as the current collectors. The positive tabs 12 are collected and connected by a weld or other attachment 14 (shown schematically). Likewise, the negative electrodes 13 have tabs 15 that extend past the edges of the positive electrodes 11 and are collected and connected by a weld or other attachment 16 (shown schematically). The tab design is simple and advantageously has only a small number of cell components. However, the tabs must be of sufficient length to reach each other to connect all the tabs and to insulate and isolate the tabs of the opposite polarity. Using tabs wastes internal volume, or headspace, around the tabs causing the battery volume to be larger or the battery capacity to be smaller than if tabs were not used. Furthermore, because the number of tabs that can be connected together in a weld or electromechanical joint is limited, this construction limits the number of electrodes capable of being stacked together. Overall, using tabs reduces the energy density of the cell. With the search for smaller and smaller packages, especially in medical applications, designers are pressed to fit more into less space, which is a difficult task with the tab design.
For stacked plate design batteries, one of the most important design requirements to ensure performance and safety is maintenance of electrode alignment. Proper alignment of the electrodes and separators must include an adequate safety margin for initial assembly and must be maintained for safe operation over the life of the cell. This is especially important for implantable cells because of the effects that a failure may have on the device performance and ultimately on the patient. For tab designs in which all of the terminations are made through a single common point, maintaining proper alignment can be difficult. Cell elements unintentionally may be allowed to rotate, leading to safety concerns such as short circuits or Li plating. While alignment is improved with multi-tab designs, headspace remains an issue, with the multiple tabs taking up significant space in the cell.
Another drawback of the typical tab design is that as the electrode layers are cut, the cutting process tends to produce burrs at the corners, particularly at inside corners such as area 17. These burrs can cause short circuits.
Another method of producing flat profile cells is by using a folded-type cell design. In these designs each electrode is layered together in an alternating fold with a separator being combined during each consecutive fold. Each panel of the electrode is connected with a small jumper ribbon of material and serves as the current collector of the entire combination of electrodes. This design is well proven in cell phone applications and has potential for inexpensive construction and manufacture. However, this geometry has size constraints and still has the problem of maintaining alignment of the components during manufacture and usage. The ribbons protrude in a similar manner as the tabs described above, thereby wasting precious space. Because the folded designs do not incorporate alignment features, the relative sizes of the electrodes are used to ensure the safety of the device. The negative electrodes are sized larger than the positive such that under its greatest misalignment, the positive is still covered by the negative. This makes the folded-type battery less space efficient.
In U.S. Pat. No. 6,139,987 to Koo et al., a bipolar battery uses an anode pin and a cathode pin to mount electrodes and contact rings, with anode contact rings fitted into cutouts of the cathode and vice versa, thus obviating the use of tabs. However, it is not clear how the electrodes are actually aligned and electrically and mechanically coupled; proper alignment and coupling are critical to the operation of the battery. Furthermore, the battery is filled with electrolyte through a center hole called an electrolyte injection hole; the electrolyte must flow into the hole and through smaller ports in the injection ring to contact and saturate the electrodes. Moreover, there is no mention of terminals and it is unclear what structures would function as terminals.
It is therefore desirable to provide a reliable flat battery having a configuration that eliminates the use of tabs for construction while overcoming other limitations of the prior art.
It is also desirable to provide a configuration that can be made into custom shapes for applications that are space-limited, such as implantable medical devices and satellites and other aerospace devices.
It is further desirable to provide a safe, compact, space-efficient, high capacity battery.